1. Field of the Invention
The present invention relates to a method of controlling current of an inverter or a multiphase inverter. More particularly, the present invention pertains to a method of controlling current of a current-controlled inverter in which, when an inductive load is controlled by pulse-width modulation, output currents of the inverter are detected, and control is effected so that instantaneous values of the output currents are substantially equal to set output current command values, respectively.
2. Description of the Related Art
Referring first to FIG. 28, which shows one type of conventional current-controlled inverter, an inverter 1 is a three-phase voltage type inverter which has a relatively small impedance when viewing the power supply side from the load. The inverter 1 is able to apply to a load 5 a total of eight kinds of voltage vector V.sub.k (k= 0, 1, 2, . . . , 7) as shown in Table 1 and FIG. 29 in accordance with the combination of ON/OFF states of switching elements defined by transistors T.sub.ra +, T.sub.ra -, T.sub.rb +, T.sub.rb -, T.sub.rc + and T.sub.rc -, that is, the combination of output potentials of the three phases of the inverter 1.
A current control circuit 3 is supplied with an instantaneous value i of the output current of each phase which is detected by current detectors 15 and 16 respectively provided on output lines of the inverter 1, and an output current command value i.sup.* for each phase which is output from an output current command value calculating circuit 4. In the current control circuit 3, a deviation .DELTA.i (.DELTA.i.sub.a, .DELTA.i.sub.b and .DELTA.i.sub.c) of the input instantaneous value i (i.sub.a, i.sub.b and i.sub.c) of the output current of each phase from the input command value i.sup.* (i.sub.a.sup.*, i.sub.b.sup.* and i.sub.c.sup.*) is obtained by means of each of the adders 6, 7 and 8, and the obtained deviation .DELTA.i is compared with a reference value by each of the hysteresis comparators 9, 10 and 11, thereby determining an output potential command for each of the phases a, b and c. The output potential commands thus determined are output to the corresponding transistors through a driver circuit 2. More specifically, output potential commands which are not inverted are employed as ON/OFF commands for the transistors T.sub.ra +, T.sub.rb + and T.sub.rc +, and output potential commands which are inverted by NOT circuits 12, 13 and 14 are employed as ON/OFF commands for the transistors T.sub.ra -, T.sub.rb - and T.sub.rc -.
TABLE 1 ______________________________________ Volt- age vec- Output potentials ON/OFF states of transistors tors a-phase b-ph. c-ph. T.sub.ra + T.sub.ra.sup.- T.sub.rb + T.sub.rb - T.sub.rc + T.sub.rc - ______________________________________ V.sub.0 - - - OFF ON OFF ON OFF ON V.sub.1 + - - ON OFF OFF ON OFF ON V.sub.2 + + - ON OFF ON OFF OFF ON V.sub.3 - + - OFF ON ON OFF OFF ON V.sub.4 - + + OFF ON ON OFF ON OFF V.sub.5 - - + OFF ON OFF ON ON OFF V.sub.6 + - + ON OFF OFF ON ON OFF V.sub.7 + + + ON OFF ON OFF ON OFF ______________________________________
It should be noted that the adder 17 obtains a detected output current value of the c-phase from detected output current values of the a- and b-phases.
The conventional current control method, which is carried out by a control apparatus such as that shown in FIG. 28, employs ever-changing instantaneous values of output currents as data on the basis of which output potentials are switched to optimal ones, which means that the response is improved in contrast to mean value current control methods such as one which employs triangular-wave comparison technique.
The above-described conventional method suffers, however, from the following disadvantage. Since an output potential is independently determined for each phase, the eight kinds of voltage vectors V.sub.k shown in FIG. 29 are selected at random, and this leads to various problems, such as increases in both switching frequency and losses, lowering in the degree of accuracy in current control, and an increase in the noise level.
In order to overcome such a disadvantage, a limited instantaneous value current control method (i.e., "Harmonic Suppressing High-Response Current-Controlled PWM Inverter Control Method", National Meeting of Electrical Engineering Society 490, 1985) has been studied in which control is effected so that an optimal voltage vector alone is selected. FIG. 30 is a block diagram of a current control apparatus which employs this method.
Referring to FIG. 30, a target voltage phase calculating circuit 20 calculates a target voltage value e.sub.0 which is represented by the following formula (1), and outputs the resultant phase angle .theta..sup.* to a voltage vector selecting circuit 18. ##EQU1## i.sup.* : output current command value i: detected output current value
L: inductance of load PA1 R: resistance of load PA1 e: internally induced electromotive force
An output current command value calculating circuit 4 calculates and outputs an output current command value i.sup.* of each phase to each of the adders 6, 7 and 8. Each adder calculates a current deviation .DELTA.i (.DELTA.i.sub.a, .DELTA.i.sub.b and .DELTA.i.sub.c) of each phase from an output current command value i.sup.* and a detected output current value i of each phase which are input thereto, and outputs the result of calculation to a current deviation quantizing circuit 19. The circuit 19 makes a comparison between the input current deviation .DELTA.i of each phase and a preset threshold value, quantizes the current deviation .DELTA.i on the basis of the the result of comparison, and outputs the quantized current deviation to the voltage vector selecting circuit 18. The circuit 18 is supplied with the phase angle .theta..sup.* of a target voltage value from the target voltage phase calculating circuit 20, the quantized current deviation from the current deviation quantizing circuit 19, and data concerning the number of times of switching which is fed back from the circuit 18 itself. On the basis of these data items, the circuit 18 calculates and outputs a voltage vector V.sub.k to be selected and data concerning the number of times of switching.
The method of selecting a voltage vector V.sub.k in the voltage vector selecting circuit 18 will be explained below with reference to FIGS. 31, 32 and 33, together with Table 2.
First, a complex plane is divided into six regions (A, B, . . . , F) every 60.degree. by the winding axes of the three phases. Then, a region in which a target voltage value is present is recognized by obtaining the phase angle of the target voltage value, and one of the switching modes (A, B, . . . , F) as shown in Table 2 is determined on the basis of the recognized region.
TABLE 2 ______________________________________ Switching modes Voltage vectors which can be selected ______________________________________ A V.sub.1, V.sub.2, V.sub.0, V.sub.7 B V.sub.2, V.sub.3, V.sub.0, V.sub.7 C V.sub.3, V.sub.4, V.sub.0, V.sub.7 D V.sub.4, V.sub.5, V.sub.0, V.sub.7 E V.sub.5, V.sub.6, V.sub.0, V.sub.7 F V.sub.6, V.sub.1, V.sub.0, V.sub.7 ______________________________________
The current deviation quantizing circuit 19 makes a comparison between the current deviations .DELTA.i.sub.a, .DELTA.i.sub.b and .DELTA.i.sub.c and a total of 15 threshold values S.sub.a1 to S.sub.a5, S.sub.b1 to S.sub.b5, and S.sub.c1 to S.sub.c5 (five values for each phase), such as those shown in FIG. 32 to detect a region in which the respective current deviation of the three phases are mutually present. The circuit 19 then quantizes the current deviations on the basis of the result of the comparison, and outputs the quantized current deviations to the voltage vector selecting circuit 18. When, for example, the quantized current deviations are present outside the outer hexagon in FIG. 32, the voltage vector selecting circuit 18 unconditionally selects the voltage vectors V.sub.1 to V.sub.6 with which the current deviations .DELTA.i are decreased most quickly, the voltage vectors V.sub.1 to V.sub.6 being set in correspondence with various regions, respectively, shown in FIG. 32. For example, when the region which is surrounded by the threshold values S.sub.a1, S.sub.b3 and S.sub.c3 is a quantizing region, the component of current deviation in the direction of the voltage vector V.sub.1 is the largest. Therefore, the voltage vector V.sub.1 is selected so that the component of current deviation in the direction of the vector V.sub.1 is decreased.
When the quantized current deviations are present inside the outer hexagon, a voltage vector V.sub.k is selected in accordance with a switching mode determined from FIG. 31 and the quantized current deviations.
When, for example, the switching mode is A, the voltage vector selecting circuit 18 selects one of the voltage vectors V.sub.1, V.sub.2, V.sub.0 and V.sub.7 in accordance with the quantized current deviations, the voltage vectors V.sub.1, V.sub.2, V.sub.0 and V.sub.7 being set in correspondence with various regions, respectively, as shown in FIG. 33. It should be noted that the inside of the inner hexagon shown in FIG. 33 involves the smallest deviation for each phase; therefore, when the quantized current deviations are present inside the inner hexagon, the voltage vector which is presently selected is not changed. In a region shown in FIG. 33 in which two voltage vectors V.sub.0 and V.sub.7 are set, either one of the voltage vectors is selected which involves a smaller number of times of switching needed when a voltage vector is changed. The judgement as to the number of times of switching is made on the basis of data concerning the number of times of switching which is fed back to the voltage vector selecting circuit 18 directly from the output thereof, said data representing the number of times of switching needed when either V.sub.0 or V.sub.7 (in the case of the above-described example) is selected.
The conventional instantaneous value current control method, employing the current control apparatus shown in FIG. 30, enables an optimal voltage vector alone to be selected on the basis of the phase angle of a particular target voltage value.
However, the arrangement shown in FIG. 30 necessitates the selection of switching modes shown in FIG. 31 to be appropriately effected on the basis of the phase angle of a particular target voltage value. Therefore, if any error in the calculation or detection of the phase angle of a particular target voltage value causes the voltage vector selecting circuit 18 to erroneously recognize the mode F when the mode A should be selected, the following problem arises. Namely, whichever voltage vector is selected from among V.sub.6, V.sub.1, V.sub.0 and V.sub.7, the direction of change of the current deviations is in the left-hand half of the complex plane from the origin 0 used as the starting point as shown in FIG. 34: that is, the range r.sub.4 for V.sub.0 and V.sub.7 ; the range r.sub.5 for V.sub.6 ; and the range r.sub.6 for V.sub.1. Accordingly, in this state, the current deviations diverge from the origin 0 to the left-hand half of the complex plane, and it is impossible to control the output currents so that they are substantially equal to the command values, respectively, by using the voltage vectors V.sub.6, V.sub.1, V.sub.0 or V.sub.7 alone.
In other words, it is necessary to accurately effect calculation and detection of the phase angle of each target voltage.
However, in order to accurately obtain the target voltage value e.sub.0 shown in the above-described formula (1) even during a transient state, it is necessary to employ an ideal sensor which is capable of detecting i and e at high speed and with high accuracy, which means that it is difficult to effect such detection in practice. In addition, since the impedance L and resistance R of the load change momentarily in accordance with temperature or other environmental factors, it is also difficult to correct them.
A delay in processing which is executed to obtain a target voltage value e.sub.0 also constitutes an error which cannot be ignored, because current deviations change at high speed.
Thus, the above-described instantaneous value current control method inevitably involves a condition in which an inappropriate switching mode is set, which means that it is difficult to effect current control by means of an optimal voltage vector alone, and this leads to various problems such as increases in the switching frequency and losses in relation to the inverter, lowering in the degree of accuracy in current control, and an increase in the noise level.
In addition, the conventional method, employing the arrangement shown in FIG. 30, may need commutation for two phases when one voltage vector is changed to another even when an appropriate switching mode is set.
For example, if the current deviations enter a region indicated by V.sub.2 in FIG. 33 when the switching mode is A and the selected voltage vector is V.sub.0, it is necessary to change the voltage vector from V.sub.0 to V.sub.2, that is, it is necessary to effect commutation for the A- and B-phases at the same time as will be understood from Table 1.
If the current deviations enter the region indicated by V.sub.1 in FIG. 33 when the switching mode is A and the selected voltage vector is V.sub.7, it is also necessary to change the voltage vector from V.sub.7 to V.sub.1, that is, it is necessary to effect commutation for the B- and C-phases at the same time.
On the other hand, if exchange between the voltage vectors alone is made as shown in the following formula (2), it must be possible to change one voltage vector to another simply by effecting commutation for only one phase EQU V.sub.0 .revreaction.V.sub.1 .revreaction.V.sub.2 .revreaction.V.sub.7 (during the mode A) (2)
Thus, the conventional method shown in FIG. 30 needs a larger number of times of commutation than that in the case of the voltage vector exchange method shown by the formula (2). For this reason, the conventional method involves the problems of increases in the switching frequency, the loss and the noise level.
The conventional method further has the following problem. When the quantized current deviations are present outside the outer hexagon in FIG. 32, oscillation occurs on any one of the threshold values S.sub.a3, S.sub.b3 and S.sub.c3. For example, when the current deviations are present outside the outer hexagon and in an area within the region of V.sub.1 which is in the vicinity of the threshold value S.sub.b3, the current deviations immediately move to the region of V.sub.2, and when the current deviations enter the region of V.sub.2, they move back to the region of V.sub.1 immediately. In other words, oscillation occurs with a period which is defined by the dead time that is required when voltage vectors are changed from one to another.